Deprecate and remove -sUSE_WEBGPU

[kainino0x]

Emscripten's implementation of webgpu.h is unmaintained. Dawn now maintains "emdawnwebgpu", a fork of Emscripten's implementation that is up to date, resolving #23432.

We want people to move to emdawnwebgpu. However, the API has changed substantially, so it will take people time to switch over. Plus, there might be new bugs in that implementation. So we need a decently long deprecation period before removing the old implementation.

Deprecation PR: #24220
Removal TBD.